The present invention relates, in general, to dielectrics used in manufacturing semiconductor devices, and more particularly, to dielectrics having low dielectric constant.
Dielectric materials are widely used in the semiconductor industry to separate metal interconnect layers from active devices formed in a semiconductor substrate. Dielectrics are also used to separate two adjacent metal interconnect layers to prevent shorting between the metal layers. Conventionally, the semiconductor industry uses chemical vapor deposited glass, spin-on glass or spin-on polyimide as dielectric materials.
One disadvantage of spin-on glass and polyimide dielectrics is their relatively high permittivity or dielectric constant. Typically, spin-on glass has a relative (to permittivity of free space) dielectric constant in excess of 3.8, while polyimides have relative dielectric constants in the range of 2.8-3.5. High dielectric constant materials produce capacitive loads on the adjacent conductors which degrades performance of both high frequency and high density transistors. Because semiconductor industry trends are towards smaller transistor structures with correspondingly smaller output drive, capacitive loading caused by interlayer dielectric materials is a mounting concern.
Another disadvantage of high permittivity dielectrics is that thicker dielectric layers are required to compensate for the high dielectric constant. Thicker layers result in larger geometry devices, increasing overall size and cost of the integrated circuit while reducing functionality. Additionally, thick dielectric layers increase planarization problems, making it difficult to form multi-layer metallizations on top of the dielectrics.
Some electronic devices which require particularly low permittivity dielectrics actually use an air gap as a dielectric material. Such devices employ what is known as an air-bridge structure, as described by Huang et al. in their article "Optimization of a Fine Line Air-Bridge Process", GaAs MANTECH, 1990 U.S. Conference on GaAs Manufacturing Technology, p. 18. Air-bridge structures usually involve providing a sacrificial dielectric layer between a conductive layer and a surface of a semiconductor device, then subsequently removing the sacrificial dielectric by etching to leave the air gap. In particular, the process of etching the sacrificial dielectric is complex, expensive and often results in residue of the sacrificial dielectric. Further, because the sacrificial dielectric must be etched after the conductive layer is patterned, air-bridge processes have limited ability to pattern the conductive layer with fine lines. Air-bridge processing would be greatly simplified and reliability of such devices greatly improved if the sacrificial dielectric etch step could be eliminated.
Accordingly, it is desirable to have a low permittivity dielectric which is compatible with semiconductor processing.